Piston for an internal combustion engine

ABSTRACT

The invention relates to a piston for an internal combustion engine, which comprises two boss bores that have respective outer areas with an oval cross-section that points on one side in the direction of the piston head. The great axis of the ovality is parallel to the piston axis and the degree of ovality increases towards the outer end of the boss bores.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claims priority under 35 U.S.C. §119 of German Application No.102 31 233.8 filed on Jul. 11, 2002. Applicant also claims priorityunder 35 U.S.C. §365 of PCT/DE2003/002289 filed on Jul. 8, 2003. Theinternational application under PCT article 21 (2) was not published inEnglish.

The invention relates to a piston for an internal combustion engine, inaccordance with the preamble of claim 1.

A piston having pin bores, each of which is divided into an inner, acenter, and an outer region, is known from DE-OS 44 31 990. In order toprevent friction contacts from forming between the pin and the pin bore,in the case of a short and rigid piston pin, as the result ofdeformation of the piston caused by great stress on the piston, i.e. bya high ignition pressure and by a great heat impact, particularly in thepin boss regions, both the inner and the outer regions of the pin boresof the known piston, in each instance, are configured as profiled pinbores. However, pistons that are under great gas pressure stress andheat stress experience a deformation in which the edge region of thepiston head bends downward in the pin direction, i.e. with reference tothe center of the piston head, particularly in the case of a shortpiston pin length, i.e. in the case of a ratio of the piston pin lengthto the piston diameter of less than 0.7. This has the result that suchgreat forces act on the zenith of the outer regions of the pin boresthat the inside walls of the pin bores come into friction contact withthe mantle surface of the piston pin there, and this can result indamage both to the inside walls of the pin bores and to the mantlesurface of the piston pin.

Proceeding from this, the invention is based on the task of avoiding pinboss friction even at greater stress on the piston.

The solution of this problem is achieved with the characteristicsaccording to the characterizing part of claim 1.

Practical embodiments of the invention are the object of the dependentclaims.

Pin bores according to the invention, having outer regions that areoval-shaped on one side or both sides, whereby the great axis of ovalitylies parallel to the piston axis, have the advantage that the zenith ofthe outer regions of the pin bores does not touch the mantle surface ofthe piston pin, so that no pin boss friction can occur here, andfurthermore, a sufficient oil reservoir always remains in existencebetween the pin bore zenith and the mantle surface of the piston pin,thereby guaranteeing good lubrication of the piston pin even in the caseof greater stress on the piston.

The invention is explained in greater detail below, on the basis of apreferred exemplary embodiment shown in the drawings. These show:

FIG. 1 a piston in cross-section, with pin bores according to theinvention,

FIG. 2 an enlarged view of the zenith of the left pin bore in partialcross-section,

FIG. 3 a cross-section through the outer segment of one of the pinbores, along the line III-III in FIG. 1, and

FIG. 4 a cross-section through the outer segment of another embodimentof the pin bores.

A piston 1 shown in cross-section in FIG. 1 is structured as a trunkpiston, in which the length of the piston pin is less than 70% of thediameter of the cylinder. In particular, this is a high-strength steelpiston as used in diesel engines. The cross-section through the pistonis selected in such a manner that the two pin bores 2 and 3 can be seen,but the piston skirt 4 is only shown in a top view.

The two pin bores 2, 3 each have an outer region 5, 5′, a center region6, 6′, and an inner region 7, 7′. The inner region 7, 7′ is configuredas a profiled bore that narrows conically from the interior of thepiston towards the outside. The center region 6, 6′ is configured incircular cylinder shape. The outer region 5, 5′ has a cross-sectionhaving an oval cross-sectional shape on one side, in the direction ofthe piston head 8, as shown in FIG. 3, which is a cross-section alongthe line III-III in FIG. 1. In this connection, a cross-section of theregion 5, 5′ having an oval cross-sectional shape on both sides, asshown in FIG. 4, is also possible. In both cases, the great axis ofovality 9 or 10, respectively, lies parallel to the piston axis 11. InFIGS. 3 and 4, a piston pin that is mounted in the pin bores isindicated with broken lines 22, 23.

The exact shape of the pin bores 2 and 3 results from the cross-sectionof the zenith 12 of the left pin bore 2, as shown in FIG. 2, the mirrorimage of which corresponds to the shape of the zenith 13 of the rightpin bore 3. The zenith of the outer region 5 is divided into threesegments 14, 15, 16 in the present exemplary embodiment, with anincreasing incline of the segments of the draft of traverse thatdelimits the three segments 14, 15, 16, towards the exterior of thepiston, and from this it is evident that the ovality of thecross-section of the outer region 5 of the pin bores 2, 3, according toFIG. 3, increases from the interior of the piston towards the outside.

The outer region 5 is followed by the center region 6, which isconfigured as a circular cylinder, and the inner region 7, which innerregion 7, configured as a profiled bore, has a circular cross-section,the radius increase of which results from the increasing incline of thetwo segments 17 and 18 of the zenith of the inner region 7.

The piston 1 according to FIG. 1 is designed in such a manner that onlya short pin is required, which hardly bends even at greater gaspressure. However, at greater gas pressure, the piston 1 experiences adeformation that is shown in exaggerated manner by means of the dottedline 19. This has the result that great forces act on the pin bores 2and 3 in the direction of the arrows 20 and 21, which forces cause adeformation of the zenith of these bores 2, 3 in a direction pointingaway from the piston head 8. Because of the fact that in this region 5,5′, the pin bores 2, 3 have an ovality that is directed upward, as shownin FIG. 3, and the piston pin, which is not shown in FIG. 1, has acircular cylindrical shape, the forces in the direction of the arrows 20and 21 merely have the effect that the ovality of the pin bores 2, 3decreases slightly in the direction of the circular shape, and therebythe distance of the zenith of the pin bores 2, 3 from the surface of thepiston pin in the region 5, 5′ decreases slightly. A remaining ovalityof the pin bores 2, 3 in the region 5, 5′ always remains, however, andas a result, for one thing, it is avoided that the zenith of the pinbores 2, 3 comes into contact with the surface of the piston pin in theregion 5, 5′, and that the piston pin is damaged due to friction on theinside walls of the pin bores 2, 3, under the influence of the forces inthe direction of the arrows 20, 21, or that the piston pin damages thepin bores 2, 3. For another thing, an oil reservoir is maintained in theregion 5, 5′, between the piston pin and the zenith of the pin bores 2,3, as a result, and this guarantees good lubrication of the piston pin,even under elevated gas pressure.

REFERENCE SYMBOL LIST

-   1 piston-   2 pin bore-   3 pin bore-   4 piston skirt-   5 outer region of 2, 3-   6 center region of 2, 3-   7 inner region of 2, 3-   8 piston head-   9 great axis of ovality-   10 great axis of ovality-   11 piston axis-   12 zenith of 2-   13 zenith of 3-   14 segment of 5-   15 segment of 5-   16 segment of 5-   17 segment of 7-   18 segment of 7-   19 dotted line-   20 arrow-   21 arrow-   22 broken line-   23 broken line

1. Piston (1) for an internal combustion engine, having a piston head(8) and having two piston pin bosses disposed on the side of the piston(1) facing away from the piston head (8), spaced apart from one another,each of which has a pin bore (2, 3) having an inner region (7, 7), acenter region (6, 6), and an outer region (5, 5), whereby the innerregion (7, 7) is configured as a conically shaped profiled bore, and thecenter region (6, 6) is configured as a circular cylinder, wherein theouter region (5, 5) has a cross-section with an oval cross-sectionalshape on one side, in the direction of the piston head (8), whereby thegreat axis of ovality lies at least approximately parallel to the pistonaxis (11), and whereby the degree of ovality increases from the borderof the center region (6, 6) to the outer end of the pin bore (2, 3). 2.Piston as recited in claim 1, wherein the degree of ovality of the outerregion (5, 5) increases in several segments (14, 15, 16), whereby eachsegment (15, 16) follows the preceding one (14, 15) without any step. 3.Piston as recited in claim 1, wherein the outer region (5, 5) has across-section having an oval cross-sectional shape on both upper andlower sides.